Rejigged solver statistics in preparation for islanding.

- See public description below.
- Stopped incrementing the iteration count in saveStats().
- `sizeof(mjSolverStat) == 40`, so this ends up costing 160KB, up from 40KB.

BEGIN_PUBLIC

Changed the size of `mjData.solver`, the structure used to collect solver diagnostic information. The array is now of length `mjNISLAND * mjNSOLVER`, where each row of length `mjNSOLVER` contains separate solver statistics for each constraint island. Until solver islanding is implemented, only row 0 is used.

- The new constant `mjNISLAND` is set to 20.
- `mjNSOLVER` is reduced from 1000 to 200.
- Added `mjData.solver_nisland`, the number of islands for which the solver ran.
- `mjData.solver_niter` (renamed from mjData.solver_iter) and `mjData.solver_nnz` are now integer vectors of length `mjNISLAND`.

END_PUBLIC

PiperOrigin-RevId: 565030093
Change-Id: I773e918805c6ced79f0dab5f19ea23956760c8c5

introspect/structs.py

@@ -3583,19 +3583,30 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  name='solver',
                  type=ArrayType(
                      inner_type=ValueType(name='mjSolverStat'),
-                     extents=(1000,),
+                     extents=(4000,),
                  ),
-                 doc='solver statistics per iteration',
+                 doc='solver statistics per island, per iteration',
              ),
              StructFieldDecl(
-                 name='solver_iter',
+                 name='solver_nisland',
                  type=ValueType(name='int'),
-                 doc='number of solver iterations',
+                 doc='number of islands processed by solver',
+             ),
+             StructFieldDecl(
+                 name='solver_niter',
+                 type=ArrayType(
+                     inner_type=ValueType(name='int'),
+                     extents=(20,),
+                 ),
+                 doc='number of solver iterations, per island',
              ),
              StructFieldDecl(
                  name='solver_nnz',
-                 type=ValueType(name='int'),
-                 doc='number of non-zeros in Hessian or efc_AR',
+                 type=ArrayType(
+                     inner_type=ValueType(name='int'),
+                     extents=(20,),
+                 ),
+                 doc='number of non-zeros in Hessian or efc_AR, per island',
              ),
              StructFieldDecl(
                  name='solver_fwdinv',